1. Field of the Invention
The present invention relates to a band-pass filter, and more particularly to a band-pass filter for use, for example, in a communication device which operates in a range of from a microwave band to a millimeter wave band, for example.
2. Description of the Related Art
Conventionally, LC filters have been widely used as band-pass filters. FIG. 17 is an equivalent circuit diagram of a conventional LC filter.
The LC filter includes first and second resonators 101 and 102. The first and second resonators 101 and 102 each include a capacitor C and an inductor L, which are connected in parallel to each other. Moreover, to form the LC filter as a single electronic component, conventionally, a monolithic capacitor and a monolithic inductor are integrated with each other in a single body. That is, two resonators each including a monolithic capacitor portion and a monolithic inductor portion are provided to define a monolithic electronic component such that the circuit arrangement shown in FIG. 17 is produced. In this LC filter, the two resonators 101 and 102 are coupled to each other via a coupling capacitor C1.
When an LC filter having the circuit configuration shown in FIG. 17 is formed as a single component, many conductor patterns and via-hole electrodes for connecting the conductor patterns must be provided. Accordingly, these conductor patterns and via-hole electrodes must be very accurately formed.
Moreover, since many electronic component elements must be formed as described above, the structure of the LC filter is complicated, and miniaturization thereof is not possible.
Furthermore, in general, the resonance frequency f of an LC filter is expressed as f=xc2xd xcfx80 (LC)1/2, in which L represents the inductance of a resonator, and C represents the capacitance thereof. Accordingly, when a relatively high frequency LC filter is produced, the product of the capacitance C and the inductance L of the resonator must be reduced. That is, for production of a high frequency LC filter, it is necessary to reduce production errors of the resonator with respect to the inductance L and the capacitance C. Thus, for development of a higher frequency LC filter, the accuracies of many conductor patterns and via-holes must be enhanced. Thus, the development of conventional high frequency LC filters is very limited.
To overcome the above-described problems with the prior art, preferred embodiments of the present invention provide a band-pass filter of which the application at a higher frequency and the miniaturization are easily realized, and of which the conditions required for control of the dimensional accuracy are facilitated.
According to preferred embodiment of the present invention, a band-pass filter includes a dielectric body, a metal film provided on the surface of the dielectric body or inside of the dielectric body, a ground electrode provided on the surface of the dielectric body or inside of the dielectric body, and opposed to the metal film via at least a portion of the layers of the dielectric body, and input-output coupling circuits coupled to first and second portions of the outer peripheral edge of the metal film, the shape and size of the metal film and the positions of the coupling points of the input-output coupling circuits being selected such that a first resonance mode of a wave being propagated in a direction that is substantially parallel to the imaginary straight line passing through the coupling points of the input-output coupling circuit, and a second resonance mode of a wave being propagated in the substantially perpendicular direction to the imaginary straight line are generated, the metal film having a protruding portion or a concavity provided thereon in the position where the resonance electric field in at least one of the resonance modes is strong, such that the first and second resonance modes are coupled to each other.
Preferably, the metal film has a substantially rectangular, substantially rhomboid, or substantially triangular shape.
Also preferably, the metal film has a substantially rectangular planar shape, and the protruding portions or concavities are provided on a pair of sides of the substantially rectangular shape.
Moreover, preferably, the metal film has a substantially rhombic planar shape, and the protruding portion or the concavity is provided on one end side of one of the diagonal lines of the substantially rhombic shape.
The features, characteristics, elements and advantages of the present invention will be clear from the following detailed description of preferred embodiments of the invention in conjunction with the accompanying drawings.